It's possible that the edge hit a small pebble that got embedded in the crotch of the tree when it was a sapling. You might not see it afterward if it was dislodged or pulverized by the chop.
Other than that, I suggest that you try one more chopping session with the blade, maybe in a more controlled environment, to see if the edge will roll again for no good reason.
Next step would be to ask Yangdu for a replacement or refund. She is very accommodating in these matters.
A huge blade like that strikes with enormous force. If you happen to hit something unusually hard, the edge might be more likely to roll than with a smaller blade that has a similar edge.